Muscle is one of the largest tissues in the body, and one that can be subjected to severe mechanical and biological stresses. A number of widespread and serious conditions cause necrosis of heart tissue, leading to unrepaired or poorly repaired damage. For example, coronary artery disease, in which the arteries feeding the heart narrow over time, can cause myocardial ischemia, which if allowed to persist, leads to heart muscle death. Another cause of ischemia is myocardial infarction (MI), which occurs when an artery feeding the heart suddenly becomes blocked. This leads to acute ischemia, which again leads to myocardial cell death, or necrosis.
Cardiac tissue death can lead to other heart dysfunctions. If the pumping ability of the heart is reduced, then the heart may remodel to compensate; this remodeling can lead to a degenerative state known as heart failure. Heart failure can also be precipitated by other factors, including valvular heart disease and cardiomyopathy. In certain cases, heart transplantation must be used to replace an ailing heart.
Unlike skeletal muscle, which regenerates from reserve myoblasts called satellite cells, the mammalian heart has a very limited regenerative capacity and, hence, heals by scar formation. The severity and prevalence of these heart diseases has led to great interest in the development of progenitor and stem cell therapy, which could allow the heart to regenerate damaged tissue and ameliorate cardiac injury. For human therapeutic application, a suitable myogenic cell type from either an autologous or appropriately matched allogeneic source may be delivered to the infarcted zone to repopulate the lost myocardium.
A number of different cell types have been considered for such therapies, including somatic cells as diverse as hematopoietic stem cells; mesenchymal stem cells; and even peripheral blood cells. Included in cells for therapy are cells derived from embryonic stem cells (ES cells). ES cells have the capacity to give rise to all tissues, including those for which no somatic stem cells are known, such as cardiac muscle (see Kehat et al. (2001) J. Clin. Invest. 108:407-414; Mummery et al. (2002) J. Anat. 200:233-242; he et al. (2003) Circ. Res. 93:32-39). ES cells have certain advantages for cardiac repair applications. There are well-defined protocols for the isolation and maintenance of ESCs, and they have a tremendous capacity for in vitro expansion, making them scalable for human applications (Zandstra et al. (2003) Tissue Eng. 9:767-778). Human ESC-derived cardiomyocytes possess the cellular elements required for electromechanical coupling with the host myocardium, such as gap and adherens junctions, and it is therefore expected that, when transplanted, these cells could electrically integrate and contribute to systolic function (see Mummery et al. (2003) Circulation 107:2733-2740). This property represents a significant advantage over other cell types, such as skeletal muscle, which act through modulation of diastolic function (see Reinecke et al. (2000) J. Cell. Biol. 149:731-740; and Reinecke et al. (2002) J. Mol. Cell. Cardiol. 34:241-249).
In brief, for human therapeutic application, a suitable myogenic cell type from either an autologous or appropriately matched allogeneic source may be delivered to the infarcted zone to replace the lost myocardium. Unfortunately, the efficacy of all potential cardiac cell therapies at present is that they are greatly limited by the subsequent death of the implanted cells.
Cell death after cardiac grafting is described by Zhang et al. (2001) J Mol Cell Cardiol 2001, 33:907-921, and similar phenomena are known to occur upon cell grafting in other tissues, for example, islet cells for diabetes (Contreras et al. (2002) Kidney Int, 61:79-84), dopaminergic neurons for Parkinson's disease (Schierle et al. (1999) Nat Med, 5:97-100), and skeletal myoblasts for muscular dystrophy (Skuk et al. (2003) J Neuropathol Exp Neurol, 62:951-967).
In cardiac engraftment, the magnitude and time course of cell death of rat neonatal cardiomyocytes implanted into the hearts of syngeneic hosts was examined by Zhang et al. Cell death was found to peak at 1 day, remain elevated at 4 days, and had largely subsided by 7 days. An estimated 90-99% of the graft myocytes had died within this first week. Importantly, increasing the number of implanted cells did not improve the outcome but instead simply increased the death rate. The underlying causes of cell death after cardiac delivery are not completely elucidated.
Methods of improving graft survival, particularly survival of progenitor cell grafts, is of great clinical interest. The present invention addresses this issue.